/**
 * @preserve
 * Copyright 2015-2018 Igor Bezkrovnyi
 * All rights reserved. (MIT Licensed)
 *
 * pngQuant.ts - part of Image Quantization Library
 */
import { AbstractDistanceCalculator } from "./distanceCalculator";

/**
 * TODO: check quality of this distance equation
 * TODO: ask author for usage rights
 * taken from:
 * {@link http://stackoverflow.com/questions/4754506/color-similarity-distance-in-rgba-color-space/8796867#8796867}
 * {@link https://github.com/pornel/pngquant/blob/cc39b47799a7ff2ef17b529f9415ff6e6b213b8f/lib/pam.h#L148}
 */
export class PNGQuant extends AbstractDistanceCalculator {
    /**
     * Author's comments
     * px_b.rgb = px.rgb + 0*(1-px.a) // blend px on black
     * px_b.a   = px.a   + 1*(1-px.a)
     * px_w.rgb = px.rgb + 1*(1-px.a) // blend px on white
     * px_w.a   = px.a   + 1*(1-px.a)
     *
     * px_b.rgb = px.rgb              // difference same as in opaque RGB
     * px_b.a   = 1
     * px_w.rgb = px.rgb - px.a       // difference simplifies to formula below
     * px_w.a   = 1
     *
     * (px.rgb - px.a) - (py.rgb - py.a)
     * (px.rgb - py.rgb) + (py.a - px.a)
     *
     */

    constructor() {
        super();
        this._setDefaults();
    }

    calculateRaw(
        r1: number,
        g1: number,
        b1: number,
        a1: number,
        r2: number,
        g2: number,
        b2: number,
        a2: number
    ) {
        const alphas = (a2 - a1) * this._whitePoint.a;
        return (
            this._colordifferenceCh(
                r1 * this._whitePoint.r,
                r2 * this._whitePoint.r,
                alphas
            ) +
            this._colordifferenceCh(
                g1 * this._whitePoint.g,
                g2 * this._whitePoint.g,
                alphas
            ) +
            this._colordifferenceCh(
                b1 * this._whitePoint.b,
                b2 * this._whitePoint.b,
                alphas
            )
        );
    }

    private _colordifferenceCh(x: number, y: number, alphas: number) {
        // maximum of channel blended on white, and blended on black
        // premultiplied alpha and backgrounds 0/1 shorten the formula
        const black = x - y;
        const white = black + alphas;

        return black * black + white * white;
    }

    protected _setDefaults() {}
}
